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We are installing a Cooktop that calls for a 40amp circuit. I was given 6AWG wire (instead of the 8AWG wire I asked for) by mistake. The electrician saw the 6AWG wire (that I had already run) and used a 50amp breaker. The Cooktop installer requires that the breaker be 40amp. My electrician was concerned that using the larger 6AWG wire would lower the resistance therefore affecting the amps; he cited Ohm's law. He has refused to change the breaker from 50amps to 40amps for fear of damaging something or affecting safety. I thought the amp requirement was dictated by the device (it asks for 40amps) and the 40amp breaker would ensure that 40amps is all that it would get. Does anyone know of any dangers/issues if using a smaller breaker with 6AWG wire?

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    Electrically there is no issue. Ohm's law has nothing to do with it. The cable is not the resistive part of the circuit. For the code part, though, see answers. – Jeffrey supports Monica Jun 11 at 15:23
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    Clearly the "electrician" doesn't understand what he's talking about. Certainly using 6 AWG vs. 8 AWG will provide lower resistance IN THE WIRING which ends up generating less heat in your walls. But that's NOT going to impact how much current the cooktop draws significantly. Putting a 40A breaker would be fine here since it's LOWER than the wire's capacity. – jwh20 Jun 11 at 15:23
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    Your electrician is wrong. He probably understands nothing about what he's doing, rather he just pattern matches: This wire = this breaker, and isn't able to actually think about it. Fire him, or monitor him closely. – Ariel Jun 11 at 23:53
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    I absolutely would not use this "electrician" anymore. If his understanding is this lacking for something so simple, imagine how much understanding he could lack in safety-critical areas.. – Trotski94 Jun 12 at 9:11
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    I am curious as to where you found this "electrician" who appears to have numerous false elementary beliefs about electricity. Is this electrician actually licensed? – Eric Lippert Jun 12 at 15:44
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Your "electrician" is not one of the brighter bulbs in the pack.

The 40A is to protect the wiring and the device.

If the wiring is AT LEAST 8Ga then it's adequate to protect the wiring. It also protects 6Ga, (or 500 MCM for that matter) just fine, and it properly protects the device at the end of the wire just fine.

"Ohms law" has squat to do with this. You could have a cooktop located 3 feet from the breaker panel and connected with 8 Ga or one located 100 feet from the panel and connected with 6 Ga - the 8 Ga would have (much, about 20 times) lower resistance, because of the wire length. Upsizing wire for longer runs on heavy circuits is actually quite normal. As stated, not a particularly knowledgeable electrician you have there.

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    But I'm sure he had no problem producing a bill for his "excellent" services. – jwh20 Jun 11 at 15:31
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    I'd say your description of the electrician is a massive understatement. If he fails to grasp the very basics of the safety principles, he's worse than useless. Who knows how many oversized breakers he's installed during his career. Most of those poor homeowners would be better off and safer if they just did it themselves. – TooTea Jun 11 at 19:17
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    He charged by the hour and actually didn't count the hour he was waiting for me to go get the 50amp breaker. He had helped us previously and his rates are decent. However, I think he may need some more time to mature on the theory vs reality aspects. He saw 6AWG and knew that normally matched up with 50amps. Based on his knowledge, straying from that could cause unknown issues. – Steve Sensabaugh Jun 11 at 20:05
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    I would have phrased this as "Do not let your "electrician" touch anything electrical in your house again. He is criminally negligent." – Martin Bonner supports Monica Jun 12 at 5:29
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    @SteveSensabaugh: Upsizing wire is at least somewhat common, though. It's not some obscure edge-case that never comes up in the real world. In fact I'd bet it's something you're tested on to get your license. – BlueRaja - Danny Pflughoeft Jun 13 at 1:07
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The breaker needs to be sized to protect the wire and the device.

Wire

Larger wire (which is a lower # due to the way wire sizes are named) can use a larger breaker. But a smaller breaker is always safe. 55A is the largest breaker you can normally use for 6 AWG copper. 40A is the largest breaker you can normally use for 8 AWG copper. But you can always use a smaller breaker - it will be 100% safe. That includes the very typical 50A (instead of 55A) for 6 AWG. But it can include lots of different things. For example, a 30A breaker on 8 AWG wire, a 15A breaker with 12 AWG wire (which can also use a 20A breaker), etc. You could even use a 15A breaker on 6 AWG wire - strange but nothing unsafe about it.

Device

The device needs to be protected by an appropriate size breaker which is determined by the design of the device and is part of the UL (or equivalent) listing for the device. So if the cooktop calls for a 40A breaker then you must use a 40A breaker. You can't use a smaller breaker (probably safe, but you would get frequent nuisance trips which are inconvenient at best and lead to unsafe operation at worst if you (or a future owner) ends up "fixing" it later in an unsafe manner). And you can't use a larger breaker because the device is not rated for that - i.e., it expects to have the protection provided by a 40A breaker in order to handle any faults in a safe manner.

It is possible to have multiple valid breaker sizes. For example, a circuit consisting of 12 AWG wire and 15A duplex receptacles can use a 15A breaker (perfect match for the individual receptacles) or 20A breaker - OK because of the wire size (15A would only need 14 AWG) and a special exception for 20A circuits that allows for multiple 15A receptacles instead of 1-or-more 20A receptacles, and the 15A receptacles are designed to allow 20A passing through. Any normal plug-in 15A device can use a 20A receptacle. But that is not necessarily the case for 40A vs. 50A - and unless the cooktop instructions actually say it is OK to do so, you need to stick with 40A, even if the wire can handle 50A.

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    Although technically true you can use 15A breaker on the 6AWG, the breaker may not be designed for a wire of that thickness, so you may actually have problems installing it properly. – Nelson Jun 12 at 3:05
  • @Nelson I realize that. I used 15A with 6 AWG as an extreme example. – manassehkatz Jun 12 at 3:25
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    This answer needs more upvotes for getting the most important point - if the appliance calls for a 40A breaker, not a 40A or higher breaker, its internal wiring may be unsafe if allowed to carry current greater than 40A under fault conditions. You absolutely need to put the 40A breaker back and fire (and ideally report) this incompetent electrician. – R.. Jun 12 at 18:59
  • This case may be from a state that doesn’t require licenses, yes they are out there, as @manassehkatz stated it would be just fine to use #6 on a small circuit, you would pigtail to the max size the breaker will allow, this is done for voltage drop all the time+. – Ed Beal Jun 19 at 13:55
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You're always allowed to upsize wire

What you have there is a 40A circuit, because it is breakered 40A per instructions.

On a 40A circuit you are allowed to use any cable 8 AWG or larger.

It's that simple.

6 AWG is larger than 8 AWG, so you are ducky-doo with the #6. Good call, since some better stoves/ranges want 50A or even 60A, and #6 is good for all that.

The only speedbump with the "any size or larger" is a very much larger wire may not physically fit on the breaker or panel lugs. In that case you need to simply pigtail to an intermediate size or metallurgy. For instance if you wisely chose 4 AWG Aluminum for your 400' long-run 30A dryer circuit, neither the 30A breaker nor socket will accept #4 nor aluminum. So you use Al-rated Polaris connectors to pigtail to #10 Cu, which will fit without trouble. #10Cu is good on a 30A circuit.

As for the electrician's "mistake" I don't see the problem. If he wasn't aware of the range specs, he made absolutely correct assumptions based on facts at hand. Many 40A ranges are dual-listed for 40 or 50A breakers, and both use the same socket. If wrong, it's a $9 change. No big.

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    With just the small caveat of 'but the wire has to fit the terminals of the breaker and device properly'. A 15 A breaker might not accept a 6 GA wire properly. I know that you know this, and that circuit breakers are labeled appropriately, but it's worth pointing out for a DIY site. – Adonalsium Jun 12 at 14:33
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    @Adonalsium good point, edited. – Harper - Reinstate Monica Jun 12 at 15:43
  • And downsize the breaker. – Agent_L Jun 14 at 8:17
  • plan for the future. Someday someone will plug in a space heater. – danny117 Jun 14 at 14:04
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The breaker now matches the wire rating (so you are protected against wall fires from overheating wires) and exceeds the cooktop rating (so it will not trigger accidentally). You are fine, but you'd have been fine with a 40A breaker as well. You state that the cooktop calls for a 40A breaker but that just means that the breaker (and wire) should be able to deliver at least 40A. The makers of the cooktop cannot delegate operating security to the wire breaker. Arguably without additional circuitry it would be a bad idea to mount the cooktop on a 5000A rail since it might melt into slag in case of a shortcircuit. The difference between 50A and 40A for the fault modes to be expected, however, are negligible. There just isn't a failure mode where the cooktop would continually draw, say, 48A through its internal wiring.

Unless, of course, some terminals have been mixed up and parts are running between different phases rather than between phase and ground, leading to a consistent too high load but not a short circuit. While the overall competence of your electrician does not seem overwhelming, his insistence on sticking with what he knows makes that unlikely. If you find that some of your plates heat water significantly faster than expected, you probably should have that double-checked.

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    Just read what you said "There isn't a failure mode" + "Unless, of course..." You can't have it both ways. – manassehkatz Jun 14 at 14:56
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You are lucky you got a 50amp circuit for your commercial kitchen. Enjoy. You can also plug a waffle iron in at the same time! Think of this as an upgrade.

OP is wrong electrician is correct using a 50amp circuit. It won't damage anything.

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    Absolutely wrong: (a) You can't plug a waffle iron (or other 120V 15A/20A appliance) into a 240V 40A/50A circuit; (b) a 40A/50A circuit isn't a duplex receptacle anyway; (c) it isn't a commercial kitchen; (d) we don't know if it "won't damage anything" without knowing the specs for the cooktop. – manassehkatz Jun 14 at 14:55
  • Two phase still an upgrade op was wrong for asking really childish I have 20 amp service to every plug in my kitchen has it damaged my 3amp coffee pot no and it never will. 50 amp is upgrade. You manassethkalf are a Hazzard to all things electric. – danny117 Jun 14 at 15:48
  • Will a 2amp phone charger die on a 10amp plug no. Becareful amps will kill you. Op got really nice upgrade. – danny117 Jun 14 at 15:49
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    is a real one. Many cooktops, ovens, etc. are designed for 40A or 50A circuit, but if they are designed/tested/listed only for 40A then if there is a fire because of a failure on a 50A circuit then insurance may deny the claim due to improper use/installation. These are real issues. – manassehkatz Jun 14 at 16:02
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    danny117, no offense but you clearly have no idea what you're talking about. You can upgrade your house service from (say) 100 amp to 200 amp, and you could even upgrade a kitchen circuit from 15 amp to 20 amp, but you cannot "upgrade" a kitchen circuit from 20 amp to 50 amp. 50 amp circuits can only be used by large appliances like ranges (stoves) and dryers. You couldn't plug a coffee maker into it if you tried, the outlets are completely different. – BlueRaja - Danny Pflughoeft Jun 14 at 16:56

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